Method and apparatus for desalination

ABSTRACT

A method and apparatus for purifying water are provided. A feed water such as seawater can be fed to a filter such as a nanofiltration membrane to produce a permeate that can, in turn, be fed to an electrodeionization system to produce fresh water.

FIELD OF THE INVENTION

The invention relates to a method and apparatus for water purificationand, in particular, to water desalination using filtration andelectrodeionization.

BACKGROUND OF THE INVENTION

A growing worldwide need for fresh water for potable, industrial, andagricultural uses has led to an increase in the need for purificationmethods that use seawater, brackish water, or other elevated salinitywater as sources. The purification of high salinity water through theremoval of dissolved solids, such as salts, has been accomplished inseveral ways including distillation and reverse osmosis (RO). Thesemethods start with a pretreated feed of seawater or other brackish waterand then purify (e.g., desalt) the water to a level that is suitable forhuman consumption or other purposes. While seawater and often, brackishwater, is a plentiful starting material, the energy required to convertit to drinking water using present RO or distillation techniques isoften cost prohibitive.

The ocean provides a limitless source of water if efficient desalinationtechniques can be developed with low environmental impact. Whileequipment cost can be high, the greatest continuing expense in desaltinghigh salinity water is energy. A small improvement in energy efficiencycan result in significant cost savings due to the large volumes of waterthat are typically processed by desalination systems.

For example, the energy required to produce potable water from seawaterby the RO process is comprised primarily of the energy that is requiredto overcome the osmotic pressure of the seawater, along with pressureloss inefficiencies during processing. Because both RO permeate and ROwastewater (often 70% of the total water fed to the system is lost towaste) must be pressurized, RO energy consumption is much higher thanthe theoretical thermodynamic minimum for desalination. Expensivemechanical pressure recovery devices are commonly needed in an attemptto recover some of the lost energy required for pressurization.

Seawater typically contains about 20,000-40,000 ppm (mg/l) of totaldissolved solids (TDS), and brackish water sources can contain from2,000 ppm to as much as 20,000 ppm TDS. These dissolved solids include avariety of monovalent, divalent, polyvalent, and/or multivalent salts orspecies, with sodium chloride typically forming about 75% or more of thetotal solids content.

While evaporative methods such as distillation have been traditionallyused to produce potable water, these methods typically require evengreater amounts of energy than do systems utilizing reverse osmosistechniques. Further these systems typically utilize complicated heatrecovery techniques to improve energy efficiency. Because RO ordistillation based processes operate at elevated pressures ortemperatures, and because high salinity water is very corrosive, exoticmetals and alloys are needed to withstand the operation conditions, andthus the need to add complicated equipment in these processes to saveenergy further increases the initial cost of the equipment and greatlydecreases the equipment reliability.

Reverse osmosis techniques can be effective at removing ionic compoundsfrom seawater. However, one serious drawback of RO systems is that ROmembranes selectively reject non-monovalent or multivalent salts to ahigher extent than monovalent salts. Thus for purification purposes inapplications such as agriculture, where most divalent ions such ascalcium and magnesium are actually beneficial for irrigation use, theseions are rejected selectively, resulting in higher than needed operatingpressures, increased potential for membrane fouling and scaling, and/orloss of valuable minerals for use in crop production.

The difference in osmotic pressure between seawater containing over 3.5%solids and potable water at less than, 1,000, or less than 500 ppm, TDSdictates that high pressures be used to produce a permeate of potablequality simply to overcome the thermodynamic free energy potential. Inpractice, since seawater is usually processed at elevated waterrecoveries to reduce pretreatment cost by reducing the amount of waterthat needs to be effectively prepared for treatment, the requiredosmotic pressure is even higher than needed to process seawater at 3.5%solids. For example, pressures utilized in RO systems are typicallygreater than 800, 900, or even 1,000 psi and for practicalconsiderations of high pressure operation, corrosion resistance,avoidance of energy losses, and prevention of scaling due to divalentselectivity and silica rejection, are limited in water recoveries (theratio of product water production to total water production) of around30% to 40%. This limitation results in a very high incremental cost ofpretreatment and water use for RO systems when it is considered that achange in water recovery from about 67% to about 33% results in adoubling of pretreatment equipment costs and a doubling of overall waterconsumption for a given pure water need. Recent advances in RO membranesand in energy reuse techniques have lowered the power consumption ofproducing potable water using RO systems to about 7 to 14 kwh per 1,000gallons (14 kwh/kgal) of water produced.

Alternative techniques using a combination of processes have alsoprovided for lower energy consumption in the conversion of seawater tofresh water. For example, two-pass nanofiltration systems have beenshown to be capable of producing potable water using a total workingpressure of about 750 psi; about 500 psi in a first stage and about 250psi in a second stage. Because energy usage relates to operatingpressure, a total working pressure of about 750 psi provides for a moreenergy efficient system compared to a typical RO system operating at apressure greater than 800 psi. See, for example, the teaching of Vuongin U.S. Patent Publication No. US2003/0205526, which is incorporated byreference herein.

In another method used to produce fresh water from seawater,nanofiltration techniques are used in conjunction with either RO orflash distillation techniques. See, for an example, the teaching ofHassan in U.S. Pat. No. 6,508,936, which is incorporated by referenceherein.

SUMMARY OF THE INVENTION

The subject matter of this application may involve, in some cases,interrelated products, alternative solutions to a particular problem,and/or a plurality of different uses of a single system or article.

In one aspect of the invention, a method for purifying water isprovided. The method comprises one or more acts of providing a feedwater comprising greater than about 1,000 ppm total dissolved solids toa nanofiltration device to produce a first filtrate reduced in totaldissolved solids, and feeding the first filtrate to anelectrodeionization device to produce a second filtrate comprising lessthan about 1,000 ppm total dissolved solids.

In accordance with another aspect of the invention, an apparatus fordesalinating seawater is provided. The apparatus comprises ananofiltration device and an electrodeionization device in fluidcommunication with the nanofiltration device.

In accordance with yet another aspect of the invention, a method fortreating seawater or other high salinity water source, to reduce totaldissolved solids is provided. The method comprises one or more acts ofpassing the source water through a water treatment apparatus whileapplying energy in an amount less than about 7 kwh/kgal of filtrate tothe apparatus and removing filtrate from the apparatus, wherein thefiltrate comprises less than about 1,000 ppm total dissolved solids.

In accordance with one or more embodiments, the invention is directed toa method of treating seawater. The method comprises acts of reducing aconcentration of one or more non-monovalent species from the seawater ina first stage and reducing a concentration of one or more monovalentspecies from the seawater in a second stage to produce water having aTDS of less than about 1,000 ppm. The first and second stages areperformed at a net energy consumption rate of less than about 7 kwh/kgalof product water.

In accordance with one or more embodiments, the invention is directed toa method of treating water having dissolved solids therein comprisingacts of reducing a concentration of monovalent dissolved species fromthe water to produce a byproduct stream, reducing a concentration ofdivalent dissolved species from the water, and reducing a concentrationof dissolved solids from the water to less than about 1,000 ppm in anelectrodeionization device.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages, features, and uses of the invention will becomeapparent from the following detailed description of non-limitingembodiments of the invention when considered in conjunction with theaccompanying drawings, which are schematic and not drawn to scale. Forpurposes of clarity, not every component is labeled, nor is everycomponent of each embodiment of the invention shown where illustrationis not necessary to allow those of ordinary skill in the art tounderstand the invention. In the drawings:

FIG. 1 is a schematic diagram illustrating a system in accordance withone or more embodiments of the invention; and

FIG. 2 is a schematic diagram illustrating a system in accordance withone or more further embodiments of the invention.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. In addition, the phraseology and terminology usedherein is for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein, is meant to encompass theitems listed thereafter and equivalents thereof, as well as additionalitems. In cases where the present specification and a documentincorporated by reference include conflicting disclosure, the presentspecification shall control.

Potable water typically has a TDS content of less than about 1,000; insome cases, potable water may have a TDS content less than about 500ppm. Examples of non-potable water are seawater or salt water, brackishwater, gray water, and some industrial water. References to seawaterherein are generally applicable to other forms of non-potable water.

Some aspects of the invention relate to methods and apparatus forpurifying seawater, and other non-potable water, which utilize, interalia, electrodesalting and electrochemical systems, and techniques thatmay be combined with pressure driven membrane systems and/or watertreatment systems. Electrochemical techniques can also include processessuch as continuous deionization, filled cell electrodialysis,electrodiaresis, and current reversing electrodialysis. As used herein,“purify” relates to reducing the total dissolved solids content andoptionally to reducing the concentration of suspended solids, colloidalcontent and ionized and non-ionized impurities in a source water to alevel where the purified water has been rendered potable and can be usedfor fresh water purposes such as, but not limited to, human and animalconsumption, irrigation, and industrial applications. Desalination is atype of purification in which salt is removed from seawater. Theinvention, in some aspects, pertains to desalination of seawater. Thefeed water or water to be treated may be from a variety of sourcesincluding those having a TDS content of between about 3,000 ppm andabout 40,000 ppm, or more. Feed water can be, for example, seawater,brackish water, gray water, industrial effluent, and oil fill recoverywater. The feed water may contain high levels of monovalent salts,divalent and multivalent salts, and organic species.

In accordance with one or more embodiments, the invention is directed toa method of treating seawater or brackish water where the source watercomprises a solute mixture wherein monovalent ions are at a higher theconcentration as compared to the concentrations of divalent and othermultivalent ions. An optional initial step comprising microfiltrationtreatment or ultrafiltration treatment is provided to remove suspendedsolids, colloidal substances and/or solutes of elevated molecularweight. Following the optional step, in this embodiment, a first stageof specialized electrodesalting treatment is provided to selectivelyremove a fraction of the monovalent ions and, following any intermediatetreatment step, is followed by a stage comprising a second membranetreatment step such as electrodeionization to provide water having adesired final purity. The overall process is capable of operating atwater recovery of from 30% to 70% or more.

Electrodeionization (EDI) is a process that removes, or at leastreduces, one or more ionized or ionizable species from water usingelectrically active media and an electric potential to influence iontransport. The electrically active media typically serves to alternatelycollect and discharge ionic and/or ionizable species and, in some cases,to facilitate the transport of ions, which may be continuously, by ionicor electronic substitution mechanisms. EDI devices can compriseelectrochemically active media of permanent or temporary charge, and maybe operated batch-wise, intermittently, continuously, and/or even inreversing polarity modes. EDI devices may be operated to promote one ormore electrochemical reactions specifically designed to achieve orenhance performance. Further, such electrochemical devices may compriseelectrically active membranes, such as semi-permeable or selectivelypermeable ion exchange or bipolar membranes. Continuouselectrodeionization (CEDI) devices are EDI devices known to thoseskilled in the art that operate in a manner in which water purificationcan proceed continuously, while ion exchange material is continuouslyrecharged. See, for example, U.S. Pat. Nos. 6,824,662; 6,312,577;6,284,124; 5,736,023; and 5,308,466; each of which is incorporated byreference herein. CEDI techniques can include processes such ascontinuous deionization, filled cell electrodialysis, orelectrodiaresis. Under controlled voltage and salinity conditions, inCEDI systems, water molecules can be split to generate hydrogen orhydronium ions or species and hydroxide or hydroxyl ions or species thatcan regenerate ion exchange media in the device and thus facilitate therelease of the trapped species therefrom. In this manner, a water streamto be treated can be continuously purified without requiring chemicalrecharging of ion exchange resin.

Electrodialysis (ED) devices operate on a similar principle as CEDI,except that ED devices do not contain electroactive media between themembranes. Because of the lack of electroactive media, the operation ofED may be hindered on feed waters of low salinity because of elevatedelectrical resistance. Also, because the operation of ED on highsalinity feed waters can result in elevated electrical currentconsumption, ED apparatus have heretofore been most effectively used onsource waters of intermediate salinity. In ED based systems, becausethere is no electroactive media, splitting water is inefficient andoperating in such a regime is generally avoided.

In CEDI and ED devices, a plurality of adjacent cells or compartmentsare typically separated by selectively permeable membranes that allowthe passage of either positively or negatively charged species, buttypically not both. Dilution or depletion compartments are typicallyinterspaced with concentrating or concentration compartments in suchdevices. As water flows through the depletion compartments, ionic andother charged species are typically drawn into concentratingcompartments under the influence of an electric field, such as a DCfield. Positively charged species are drawn toward a cathode, typicallylocated at one end of a stack of multiple depletion and concentrationcompartments, and negatively charged species are likewise drawn towardan anode of such devices, typically located at the opposite end of thestack of compartments. The electrodes are typically housed inelectrolyte compartments that are usually partially isolated from fluidcommunication with the depletion and/or concentration compartments. Oncein a concentration compartment, charged species are typically trapped bya barrier of selectively permeable membrane at least partially definingthe concentration compartment. For example, anions are typicallyprevented from migrating further toward the cathode, out of theconcentration compartment, by a cation selective membrane. Once capturedin the concentrating compartment, trapped charged species can be removedin a concentrate stream.

In CEDI and ED devices, the DC field is typically applied to the cellsfrom a source of voltage and electric current applied to the electrodes(anode or positive electrode, and cathode or negative electrode). Thevoltage and current source (collectively “power supply”) can be itselfpowered by a variety of means such as an AC power source, or forexample, a power source derived from solar, wind, or wave power. At theelectrode/liquid interfaces, electrochemical half cell reactions occurthat initiate and/or facilitate the transfer of ions through themembranes and compartments. The specific electrochemical reactions thatoccur at the electrode/interfaces can be controlled to some extent bythe concentration of salts in the specialized compartments that housethe electrode assemblies. For example, a feed to the anode electrolytecompartments that is high in sodium chloride will tend to generatechlorine gas and hydrogen ion, while such a feed to the cathodeelectrolyte compartment will tend to generate hydrogen gas and hydroxideion. Generally, the hydrogen ion generated at the anode compartment willassociate with a free anion, such as chloride ion, to preserve chargeneutrality and create hydrochloric acid solution, and analogously, thehydroxide ion generated at the cathode compartment will associate with afree cation, such as sodium, to preserve charge neutrality and createsodium hydroxide solution. In accordance with further embodiments ofthis invention, the reaction products of the electrode compartments,such as generated chlorine gas and sodium hydroxide, can be utilized inthe process as needed for disinfection purposes, for membrane cleaningand defouling purposes, and for pH adjustment purposes.

In accordance with some embodiments of the invention, a plurality ofstages in a treatment system can be utilized to purify water or at leastreduce the concentration of dissolved solids therein. For example, waterto be treated can be purified in stages such that each stage selectivelyremoves one or more types of dissolved solids thereby producingpurified, e.g., desalted, or even potable, water. In some cases, one ormore stages can comprise one or more unit operations that effectsselective retention of a type of dissolved species, which can then beremoved in one or more subsequent or downstream stages utilizing one ormore other unit operations. Thus, in some embodiments of thepurification system of the invention, a first stage can remove or atleast reduce the concentration of one type of dissolved species. Inother embodiments, the first stage can remove or reduce theconcentration of all but one type of dissolved species. Any retainedspecies, not removed from the water, can then be removed or theconcentration thereof reduced in one or more subsequent stages.

Some embodiments of the invention relate to aspects that advantageouslyutilize byproduct streams from one or more stages to effect regenerationor recharging of one or more other stages. A dischargeable stream orbyproduct stream from one or more stages of the system of the inventioncan have a high concentration of a first dissolved species removed fromthe water to be treated. The presence of the first dissolved species insuch a stream can facilitate regeneration of other unit operations inone or more other purification stages. For example, an electrodialysisstage can remove or reduce the concentration of monovalent species fromseawater. For example, Table 1 provides concentrations of primarytypical solutes found to make up the salts comprised in a typicalseawater. Based on those constituents and assuming about 80% overall TDS(total dissolved solids) removal in a first stage operating at about 67%water recovery, comprising monovalent selective anion and cationexchange membranes, the solute makeup of the depleting and concentratingstream effluent from the stage as a function of membrane selectivitycoefficient can be determined. Membrane selectivity coefficient can bedefined as

${Selectivity} = \frac{\frac{\Delta\; v_{Na}}{v_{Na}}}{2\left\lbrack \frac{{\Delta\; v_{Ca}} + {\Delta\; v_{Mg}}}{v_{Ca} + v_{Mg}} \right\rbrack}$

where ν is the molarity of ionic species i and Δν is the change in themolarity of the ionic species. Table 2 provides calculated values ofsolutes remaining in the ion depleting stream and ion concentratingstream effluents from a first stage separation apparatus comprisingmonovalent selective anion and cation membranes with selectivities of 1(non-selective), 5, and 10. The data in Table 2 were derived for aproduct water with about 20,000 ppm TDS and an assumed recovery rate ofabout 67%.

TABLE 1 Seawater Typical Composition. Concentration Species Ppm Chloride19,000 Sulfate 2,700 Bromide 65 Silicate 3 Iodide 0.06 Phosphate 0.07Sodium 16,500 Magnesium 1,350 Calcium 400 Potassium 380 Lithium 0.17Boron 4.6 Strontium 8 Molybdenum 0.01 Manganese 0.002 Aluminum 0.01Cadmium 0.00011 Chromium 0.00005 Cobalt 0.0004 Copper 0.003 Iron 0.06Lead 0.00003 Nickel 0.007 Selenium 0.00009 Silver 0.0003 Zinc 0.01

TABLE 2 Depleting and concentrating Stream properties using softenedseawater into 2-stage ED devices. Monovalent Double Non-SelectiveSelective ED Selective ED ED Ca Ca Ca concentration, concentration,Concentration, mmol/L LSI mmol/L LSI mmol/L LSI 5.1 1.08 2.63 0.79 17.781.62

As can be seen in Table 2, for devices comprising monoselectivemembranes, the concentrations of solutes such as calcium, magnesium, andsulfate, which tend to cause fouling and scaling of the concentratingcompartments of the device, are maintained at relatively lowconcentration levels in the concentrating stream relative to devicesutilizing comprising nonselective membranes. The result is that use ofmonovalent selective membrane devices enables increased water recoverywithout causing salt precipitation and resulting performance loss, orplugging of the desalting device. Monovalent selectivity may notnecessarily disproportionately lower bicarbonate levels in theconcentrating stream, but the potential for precipitation of bicarbonatecompounds such as calcium bicarbonate is nevertheless reduced because ofthe disproportionate lowering of calcium levels (e.g., relative tosodium) in the concentrating stream. In addition, as will be discussedin more detail, acidic electrolyte products from the use of highsalinity sodium chloride as an electrolyte can be used as a reagent feedto the concentrate stream, to adjust and lower the pH of the concentratestream and thus inhibit the potential of any residual calciumbicarbonate in the concentrate stream to form scale, by shifting thebicarbonate equilibrium away from the carbonate form.

The byproduct stream (e.g., the concentrate stream of a monoselective EDstage) would have a high concentration of such species, e.g., sodiumchloride, which can then be utilized to facilitate regeneration of anion exchange unit operation that may then optionally be utilized toselectively remove or reduce the concentration of dissolved divalentspecies from the water to be treated. Moreover, where further stagesincluding other types of unit operations are utilized to further removeor reduce the concentration of remaining species and/or trace impuritiesfrom a fraction of, or all the depleting stream, so that problematicconstituents that remain in the depleting stream effluent of the firststage are selectively removed before end use (e.g., boron removal viaselective ion exchange prior to being provided for agriculturalirrigation water) or prior to being fed to a second membrane state ofthe overall system (e.g. calcium and magnesium removal via chemicallyregenerable cation exchange to avoid plugging and scaling in the secondmembrane stage).

By placing the optional ion exchange unit downstream of the firstmonoselective removal stage there is additional process advantage withrespect to operation of the ion exchange unit. Operation of an ionexchanger, e.g., a cation exchanger for removal of calcium and magnesiumfrom a source water, is much less efficient in its removal capability ifthe source water is high in overall salinity. Thus, by operation of theion exchanger downstream of the first salt removal stage, whereby alarge fraction of the salts are already removed compared to the sourcewater, the ion exchanger will operate more efficiently and producebetter quality effluent with less chemical need for regeneration.

Moreover, where further stages including types of units of operationsare utilized to further remove or reduce the concentration of remainingspecies from the water stream, any byproduct streams therefrom can alsobe utilized to facilitate regeneration of one or more other unitoperations in the other stages.

Other aspects of the invention can be considered as being directed toreducing the overall byproduct or waste discharge burden associated withpurification of the water to be treated. Indeed, a byproduct stream fromone or more stages or unit operations can be directed to one or moreupstream or downstream stages or unit operations and be combined withwater to be treated and/or be utilized to facilitate the operation ofsuch unit operations.

In accordance with one or more aspects of the invention, EDI systems andtechniques, including CEDI systems, can be combined with one or moretechniques to purify non-potable water, for example water having greaterthan about 5,000 ppm TDS, to produce potable water. In accordance withone or more embodiments of the invention, one or more states comprisingpressure-driven separation techniques such as filtration to remove aportion of the TDS in water from a non-potable water source, and one oremore electrically-driven separation techniques such aselectrodeionization to remove an additional portion of the TDS in thefirst filtered water to eventually produce potable water. Thepressure-driven separation technique, in some cases, can be based onnanofiltration (NF) systems and techniques. In accordance with otherembodiments, electrically-driven separation techniques such as, but notlimited to, electrodialysis or electrodiaresis, can be utilized with,for example, filtration and/or EDI systems and techniques to purify,e.g., desalinate, water. Further embodiments of the inventioncontemplate utilizating combinations of such systems and techniques.Thus, for example, electrodeionization systems can be utilized with twoor more systems that, in combination, preferentially remove one or moretypes of dissolved solids. Indeed, in accordance with one or moreembodiments of the invention, an electrodeionization stage can beutilized with an electrodialysis stage and/or an ion exchange stage.

Nanofiltration techniques can be used to remove species smaller thanthat which can be removed by ultrafiltration (UF) techniques, buttypically does not remove all species that can be removed by reverseosmosis techniques. Nanofiltration membranes can incorporate both stericand electrical effects in rejecting or selectively separating dissolvedspecies. Thus, for example, nanofiltration membranes may also remove orreduce the concentration of uncharged organic molecules including, forexample, organic molecules having a molecular weight of greater thanabout 150 Daltons or, in some cases, greater than about 300 Daltons.Divalent and/or multivalent ions are typically removed at a rate ofgreater than about 90%. However, in some cases, greater than about 95%;and in some applications, greater than about 98% of the multivalentspecies can be removed by such selective techniques. Nanofiltrationsystems, however, are typically less efficient at removing monovalentions than divalent or non-monovalent ions and may remove, for example,less than about 10%, less than about 25%, less than about 50%, less thanabout 75%, or less than about 90% of the monovalent ions present in afeed water to be treated. Nanofiltration membranes may be made from avariety of materials, including, for example, polyamide materials. See,for example, U.S. Pat. Nos. 6,723,241 and 6,508,936 as well as U.S.Patent Pub. No. 2003/0205526; each of which is incorporated by referenceherein.

As noted, in some instances nanofiltration systems and techniques maynot remove monovalent ions efficiently or at least to an extent that isdesirable and/or economically practical. Seawater, however, typicallyhas a TDS content that is about three quarters in the form of monovalentsalts. Table 1 lists the typical concentration of various, but notnecessary all, species in seawater.

The associated operating pressure required to treat water utilizingnanofiltration membranes can be significantly less than the operatingpressure required to pass water through RO membranes, where themonovalent salts contribute greatly to the difference in osmoticpressure between the feed and the permeate. Depending on theapplication, feed water may be purified in a nanofiltration device at anoperating pressure of less than about 600 psi; in some cases, at anoperating pressure of less than about 500 psi; or in other cases, at anoperating pressure of less than or equal to about 400 psi. The permeateresulting therefrom typically is reduced in organic speciesconcentration and divalent and nonmonovalent ions concentration bygreater than about 90%, while retaining more than about 10% of themonovalent ionic constituents. Depending on the specific configurationand the feed water, more that about 25% of the monovalent ions areretained or retrieved; in some applications, more than about 50% of themonovalent ions are retrieved; and in other applications, more thanabout 75% of the monovalent ionic constituents are retrieved. Therefore,a nanofiltration device having seawater, brackish water, or feed waterhaving similar composition, can provide a filtrate that is substantiallyreduced in divalent and non-monovalent ionic constituents, and/ororganic constituents but may retain a significant portion of the initialmonovalent ion constituents, such as, sodium chloride. The filtrate,when compared to the feed, may exhibit a reduction in TDS of greaterthan or equal to about 30% (in some cases, up to and including about95%). In most cases, however, a one-pass nanofiltration filtrate fromseawater is unsuitable for human consumption or use in applicationsrequiring fresh water and further treatment may be required to renderthe water suitable.

In accordance with one set of embodiments of the invention, the productsuch as the filtrate from a nanofiltration stage is fed to anelectrodeionization stage (such as a CEDI device). The divalent andmultivalent cations, such as magnesium and calcium, which typicallycontribute to hardness may, in a large extent, be removed in thenanofiltration stage or an intermediate ion exchange softener downstreamof a monovalent selective ED stage. Electrodeionization devices can, inturn, remove monovalent cations and/or anions, such as sodium chlorideand further, operate at lower power consumption on feed waters devoid ofdivalent ions. Thus, a feed water that contains TDS of primarilymonovalent salts can be efficiently purified by passing the waterthrough one or more electrodeionization devices and one or morenanofiltration devices. One or more passes at each stage may be employedand two or more electrodeionization devices can be used in series or anysuitable arrangement. Typically, but not necessarily, the nanofiltrationstages precede the electrodeionization stages. Such configurations canlead to a decrease in, or even an absence of, fouling of downstream unitoperations and/or components, such as in the concentration compartmentsand associated housing assemblies of an electrodeionization device aswell as, fittings and conduits. Therefore, one or more nanofiltrationdevices can be advantageously used to remove divalent and/or multivalentions, such as hardness-causing species, and one or moreelectrodeionization devices can be advantageously used to removemonovalent ions, thus reducing or eliminating fouling tendencies.Indeed, the invention can be directed to systems and techniques thatprovide one or more stages or unit operations that can remove, or atleast reduce the concentration of, one or more divalent and/ormultivalent species from a water stream; and one or more stages or unitoperations that can remove, or at least reduce the concentration of, oneor more monovalent species from the water stream. The resultant productwater can thus be rendered potable.

Other aspects of the invention are directed to systems and techniques ofpurifying a water stream by reducing, or even minimizing, a tendency ofone or more species in a water stream to form scale or foul the membranedevices, in a first stage, or preliminary stages, and by removing, or atleast reducing a concentration of monovalent species, in a second stageor subsequent stages.

The first stage, e.g., filtration such as nanofiltration, can beoperated at an energy requirement of less than or equal to about 4.7kwh/kgal. The second stage, e.g., by electrochemical, such aselectrodeionization, can be operated at less than or equal to about 2.3kwh/kgal. Thus, an overall energy usage of about 7 kwh/kgal can beexpected, which provides a significant decrease in energy consumptionwhen compared to other desalination techniques, such as evaporative, RO,two-pass nanofiltration or hybrid nanofiltration/RO, andnanofiltration/evaporative techniques.

In accordance with one or more embodiments of the invention, retentate(reject) and concentrate containing fluids that result from the process,typically containing greater amounts of TDS than their respective feedwaters, can be discharged to the feed water source or to conventionalwastewater treatment facilities. Concentrate effluent from, for example,a CEDI device, can be recycled as feed to or combined with feed water toa nanofiltration device. In some cases, for example when a concentratedbrine is produced from the concentrate compartments of a CEDI device,the brine, which may be substantially or essentially free of divalentand multivalent ions, or have a reduced level of scale-forming species,can be used as a brine source for the production of a disinfectant, suchas, but not limited to, sodium hypochlorite. The softened brine solutioncan provide a source of electrolyzable chlorine species for use in adisinfectant-forming system which can utilize, for example, anelectrolytic device. Thus, if purified water produced utilizing someaspects of the invention can benefit from later disinfection, a readysource of softened, concentrated brine, and/or disinfectant, can beavailable at low cost.

Prior to treatment of feed water, a variety of pre-treatment procedurescan be employed. For example, pretreatment techniques may be utilized ona feed water that may contain solids or other materials that mayinterfere with or reduce the efficiency of any stage or device, such asthe nanofiltration device or the EDI device. Pretreatment processes maybe performed upstream of the nanofiltration device and/or the EDI deviceand may include, for example, particulate filtration, sand filtration,carbon filtration, microfiltration, such as cross-flow microfiltration(CMF), combinations thereof and other methods directed to the reductionof particulates. Adjustments to the pH and/or alkalinity of feed watermay also be performed by, for example, the addition of an acid, base orbuffer, or through aeration.

A particularly important optional advantage of the embodiment comprisinga first monovalent selective stage is that because the water recovery ofsuch a system is higher than possible using present technology, theamount of pretreatment required to process the source water is greatlydiminished. Thus the amount of needed pretreatment equipment isdecreased proportionately. The result is a reduced cost and size ofpretreatment equipment, and/or alternately, enables the implementationof pretreatment systems that would not normally be consideredeconomically feasible. For example, membrane microfiltration, atechnique that removes not only bulk particulates, but also microbialcontaminants and other harmful colloidal constituents in a source water,can be used more cost-effectively as a substitute for traditional andless effective sand filtration systems. This improves end use waterquality while increasing the reliability of downstream treatmentcomponents.

One embodiment of a system of the invention is illustrated in FIG. 1.System 100 includes one or more nanofiltration devices 110 as well asone or more electrodeionization devices 120. Nanofiltration device 110comprises a nanofiltration membrane disposed in a housing.Electrodeionization device 120 comprises one or more anode, cathode,concentration, and depletion compartments. Water sources are providedfor depletion, concentration, and electrode compartments ofelectrodeionization device 120. A feed water source 140 may be, forexample, the ocean. Feed water can be pumped through conduit 150 and bepressurized by pump 130 to pass through the nanofiltration membrane innanofiltration device 110. Typically, pump 130 pressurizes the feed toan operating pressure of about 600 psi or less. Permeate fromnanofiltration device 110, reduced in multivalent ionic content, passesthrough conduit 160 as a feed stream to electrodeionization device 120.Reject fluid from device 110 passes through conduit 170 and may bedirected, for example, back to the feed water source 140.

Energy may be recovered from the retentate stream and used, for example,to heat feed water, provide electricity, and/or to provide energy forother processes or unit operations, which need not be directlyassociated with system 100. Water in conduit 160 may be fed to any ofthe depletion, concentration, and/or electrode compartments ofelectrodeionization device 120. Electrodeionization device 120 istypically powered by a source of electric current (not shown), which maybe configured to provide a reversible electric field. Purified diluentis received at conduit 180 where it may be sent for use or storage aspotable water. Potable water may be preserved or further disinfected, ifdesired. A concentrate stream from electrodeionization device 120 may bedischarged to waste via conduit 190, may be recycled through the systemvia conduit 192, or may be used as a source of brine via conduit 194.The concentrate stream may have a sodium chloride content similar tothat of seawater and can be a source of feed water to nanofiltrationdevice 110.

The systems and techniques of the invention may be operated on acontinuous or a batch basis, and may be operated at a fixed location oron a mobile platform, such as on board a vessel or on a vehicle.Multi-pass CEDI systems may also be employed wherein feed is typicallypassed through the device two or more times, or may be passed through anoptional second device. In some cases, the electrodeionization devicemay be heated to, for example, increase the rate of ionic speciestransport therein. Thus, the electrodeionization device may be operatedat ambient temperature; alternatively, the electrodeionization devicemay be operated at a temperature greater than about 30° C., greater thanabout 40° C., or even greater than about 50° C.

In some cases, it may be desirable to reduce the internal electricalresistance of the electrodeionization device to minimize energy usage.Therefore, in accordance with one or more embodiments of the invention,low electrical resistance membranes may be used to separate or definedepletion and/or concentration compartments thereof. For example,individual compartments, or cells of the electrodeionization device, maybe constructed to have a width of less than about 10 millimeters. Theuse of low electrical resistance membranes and/or thin compartments canhelp to reduce electrical resistance or load and, therefore, serve todecrease electrical power requirements. Low electrical resistancemembranes that may be utilized in accordance with some embodiments ofthe invention include, for example, those commercially available asNEOSEPTA® membranes, from ASTOM Corporation, Tokyo, Japan. In someembodiments of the invention, intermembrane spacing may be, for example,less than about 0.1 inch, less than or equal to about 0.06 inch, or lessthan or equal to about 0.05 inch.

In some applications, it may be important or desirable to reduce theconcentration of boron species in a water to a level that is consideredto be suitable for agricultural service and/or human consumption. Forexample, the desired concentration of boron species can desirably beless than about 1 ppm. In some cases, the concentration of boron speciesis desirably about or even less than the level as suggested bygovernment and/or health organizations. For example, the concentrationof boron can be at about or less than the level suggested by the WorldHealth Organization, at about 0.5 ppm. Indeed, in some applications, theconcentration of boron in the treated water is preferably less thanabout 0.4 ppm.

Because seawater often contains high levels of boron, for example, about1 to about 4 ppm, this target, recommended or suggested boron level canbe difficult to achieve utilizing conventional desalination processes.Advantageously, the systems and techniques of the invention can beutilized to reduce boron species concentration in feed water to anacceptable level. Indeed, some embodiments of the invention are directedto systems and techniques that reduce the boron concentration in a feedstream from about 4.6 ppm to less than about 0.5 ppm.

In addition to lower energy costs, the systems and techniques of theinvention can provide lower capital, operating, and/or maintenancecosts. For example, due to an ability to operate at lower operatingpressures, lower cost materials, such as plastic piping, can be employedin the systems of the invention, instead of high pressure stainlesssteel and/or titanium alloys that are typically necessary in RO systems.

To purify seawater, the water needs to be separated from its dissolvedcomponents. The energy required to perform this separation is the amountof energy required to overcome the difference in osmotic pressurebetween the feed water (seawater) and the product (fresh water).

The osmotic pressure of seawater, P_(sw), can be determined from thevan't Hoff equation: P_(sw)=c*R*T, where c is the ionic molarconcentration, R is the gas constant, 0.082 liter-bar/degree-mole, and Tis the absolute temperature in Kelvin, T=300 K (about 27° C.). The ionicsalt concentration in seawater is about 1.12 mole/liter, assuming a puresodium chloride solution. Therefore, the osmotic pressure is determinedto be about 400 psi (1.12*0.082*300=27.6 bar).

Desalination energy requirements are typically provided per 1,000gallons of purified water per hour. An estimate of the theoreticalminimum energy required to desalinate seawater is about 2.9 kwh/kgal (orabout 0.77 kwh/m³), determined as follows, assuming about 400 psi oftrans-membrane pressure (for the NF membrane) and 100% pump efficiency:

${{Brake}\mspace{14mu}{horsepower}} = {\frac{({gpm})({psi})}{(1715)({eff})} = {\frac{(16.67)(400)}{(1715)(l)} = {3.89\mspace{14mu}{{bhp}\left( {2.9\mspace{14mu}{kw}} \right)}}}}$

The method used (thermal or pressure driven) to desalinate seawater isbelieved to be independent from the minimum energy required.

Example 1

A comparison of state of the art RO-based systems with the systems andmethods of the invention illustrate the energy savings that areachievable. A conventional RO-based system requires about 19.2 kwh/kgalto desalinate seawater as shown by the following calculations. In thecalculations, the assumed trans-membrane pressure is about 900 psi, theassumed pump efficiency is about 85%, and the assumed water recovery isabout 40%. Further, for an inlet flow rate of about 41.67 gpm, about16.67 gpm of permeate and about 25 gpm of reject is produced. No energyrecovery techniques are assumed to be utilized.

${{Brake}\mspace{14mu}{horsepower}} = {\frac{({gpm})({psi})}{(1715)({eff})} = {\frac{(41.67)(900)}{(1715)({.85})} = {25.73\mspace{14mu}{{bhp}\left( {19.2\mspace{14mu}{kw}} \right)}}}}$

However, if energy recovery techniques are utilized, the calculatedenergy requirement can be reduced. Examples of energy recoveryassemblies or techniques include, for example, a turbine, such as aPelton wheel, or a pressure exchange device. Both types of devices canrecover the energy from the high-pressure reject stream and transfer theenergy to the feed stream of the RO device. A Pelton wheel assembly istypically about 80% efficient in recovery and positive displacementsystems typically claim recovery efficiencies of about 90% to about 95%.

To calculate the effect of energy recovery on the overall powerconsumption, about 40% of the power is assumed as being consumed in thepermeate stream (0.4*19.2 kwh/kgal=7.68 kwh/kgal), and about 60% of thepower is assumed as being consumed in the reject stream (0.6*19.2=11.52kwh/kgal). Assuming that, for example, about 93% of the energy in thereject stream can be recovered, about 7% is thus consumed(0.07*11.52=0.81 kwh/kgal). Therefore, the overall power consumption ofan RO device, utilizing energy recovery techniques, is about7.68+0.81=8.49 kwh/kgal.

Example 2

To estimate the amount of total energy required to desalinate seawaterutilizing a system comprising a combination of nanofiltration andelectrodeionization devices, the energy requirements for each areindependently determined and then combined.

The energy requirement associated with the NF device is approximated tobe about 10.7 kwh/kgal (about 2.8 kwh/m³) as shown by the followingcalculation.

${{Brake}\mspace{14mu}{horsepower}} = {\frac{({gpm})({psi})}{(1715)({eff})} = {\frac{(41.67)(500)}{(1715)(0.85)} = {14.3\mspace{14mu}{{bhp}\left( {10.7\mspace{14mu}{kw}} \right)}}}}$

This estimate was derived by assuming that the trans-membrane pressurewas about 500 psi, the pump efficiency was about 85% pump efficiency,and the water recovery was about 40%. This estimate was further based onan inlet flow rate of about 41.67 gpm, producing about 16.67 gpm ofpermeate and about 25 gpm of reject. No energy recovery techniques wereutilized.

Energy techniques can also be utilized in nanofiltration devices in amanner similar to that described above with respect to RO-based devices.Further, similar assumptions are utilized with respect to recoveryefficiencies: about 40% of the power is assumed as being consumed by thepermeate stream (0.4*10.7 kwh/kgal=4.28 kwh/kgal), and about 60% of thepower is assumed as being consumed by the reject stream (0.6*10.7=6.42kwh/kgal). If about 93% of the energy in the reject is recovered, thenabout 7% is assumed as being consumed (0.07*6.42=0.45 kwh/kgal). Thus,the power consumed associated with the nanofiltration device would beabout 4.73 kwh/kgal (4.28+0.45=4.73 kwh/kgal).

To consume less energy than an RO system, a desalination systemcomprising nanofiltration and CEDI stages would, in aggregate, need toconsume less energy than the RO system alone. As shown above, thenanofiltration stage consumes about 4.7 kwh/kgal while the total energyconsumption of the RO system is about 8.5 kwh/kgal. Thus, to exhibittotal energy consumption below that of the RO system, the powerconsumption of the CEDI stage is preferably less than or equal to about3.8 kwh/kgal.

If the nanofiltration system rejects about 91% of the inlet totaldissolved solids contained in seawater feed, the downstream CEDI modulewould preferably remove about 90% of the remaining TDS in order for thewater to meet drinking water standards of less than 500 ppm TDS. To becompetitive with RO systems, therefore, the CEDI module may need toremove this amount of solids using less than about 3.8 kwh/kgal of waterproduced.

Example 3

A system was operated to determine if seawater can be purified(desalinated) to a level of less than about 500 ppm TDS. The system wascomprised of a nanofiltration device and CEDI devices that meets theabove-mentioned energy requirement (less than about 3.8 kwh/kgal).Artificial seawater was prepared from INSTANT OCEAN® synthetic sea saltmix, available from Aquarium Systems, Mentor, Ohio.

The nanofiltration and CEDI devices were operated under the followingconditions:

Closed loops were utilized for both nanofiltration and CEDI devices.Electrode compartment feed water for the CEDI device, which was separatefrom the nanofiltration product water, was made up from RO water, withH₂SO₄ added to achieve a pH of about 2. Feed water calcium content wasapproximately 50 mg/l, measured as CaCO₃.

The nanofiltration device utilized a FILMTEC® NF90 (4×40) membrane,available from The Dow Chemical Company, Midland, Mich. The inlet waterstream into the nanofiltration device was pressurized to about 500 psiand had a conductivity of about 45.5 mS/cm. The permeate from thenanofiltration device had a conductivity of about 4.2 mS/cm at a flowrate of about 3.25 l/m. The reject flow rate was about 36 l/m. Thenominal operating temperature of the device was about 23° C.

Two different CEDI devices were evaluated, designated as Stack A (lowerelectrical resistance) and Stack B (standard). Each of Stacks A and Bwere comprised of 20 cell pairs in a 2-stage folded path, with 10 cellpairs in each stage. The flow path length was about 28 inches. Bothstacks also utilized an iridium oxide-based anode, a stainless steelcathode, and an about 50/50 mix of DOWEX™ MARATHON strong baseanion/strong acid cation resins, both from The Dow Chemical Company,Midland, Mich. The inter-membrane spacing of each of Stacks A and B wasabout 0.06 inch. Stack A included alternating ion exchange membranes.

The operating conditions and performances of both modules is summarizedin the Table 2, below. The energy demand data listed in Table 2 does notconsider the power supply efficiency.

The data shows that Stack A is preferred over stack B because the formerproduces similar quality product at a similar rate while using about 40%less energy.

Thus, assuming that the nanofiltration device requires about 4.7kwh/kgal to achieve the desired performance of less than about 90%removal, the system comprising nanofiltration and CEDI devices wouldyield a power consumption of about 7 kwh/kgal. This overall energyrequirement is about 15% less than the energy requirement of aconventional RO-based system.

TABLE 3 Operating and Measured Parameters of CEDI Stacks A and B.Operating or Measured Parameter Stack A Stack B Flow Rate through theDilution Compartment (ml/min) 280 280 Flow Rate through theConcentration 72 73 Compartment (ml/min) Flow rate through the ElectrodeCompartment, 250 200 (ml/min) Feed Stream Conductivity (mS/cm) 4.2 4.2Product Conductivity (μS/cm) 570 550 Dilution Compartment Pressure Drop(psi) 5.6 7.5 Concentration Compartment Pressure Drop (psi) 2.2 3.6Electrode Compartment Pressure Drop (psi) 6.4 8.9 Applied ElectrodePotential (VDC) 17.15 26.6 Cell Pair Voltage (VDC) 13.5 22.0 Voltage perCell Pair (VDC) 0.675 1.1 Current Consumption (A) 0.84 0.83 EnergyConsumption (kwh/kgal) 2.5 4.2 Operation Duration (hours) 175 274Product TDS (ppm) 240 232

Example 4

This example describes further embodiments of the invention that can beutilized to reduce the concentration of dissolved solids in seawater.

As illustrated in FIG. 2, the system can comprise at least oneelectrodeionization stage disposed downstream of one or more monovalentspecies-reducing stages and one or more divalent species-reducingstages.

The monovalent reducing stage can comprise any unit operation thatreduces the concentration of monovalent species such as, but not limitedto, sodium chloride. Examples of unit operations that can serve toreduce the concentration of monovalent dissolved solids include, but arenot limited to, electrodialysis and electrodiaresis devices.

This monovalent species reducing stage can operate at elevated waterrecovery, for example, about 60% to 70% or higher depending on theselectivity coefficient of the membrane with respect to monovalentspecies versus non-monovalent species. This is due to the avoidance ofthe potential for nonmonovalent or multivalent species to scale or foulthe membrane devices, since their concentration does not increase in thesame proportion as monovalents species. Such a device is much less proneto fouling and scaling relative to other processes such as non-selectiveED, or distillation, and very much less prone to fouling and scalingrelative to processes such as NF and RO, which selectively concentratemultivalent species and foulants over monovalent species. By operatingat elevated water recovery, not only is the process more efficient by,for example, reducing the volume requirements for pretreatment equipmentand materials, but also the total amount of water required for theoverall process is reduced, which is of particular importance in regionswith scarce water resources. In addition, by operating at high waterrecovery, the concentration of salt in the concentrate stream of thedevice is increased, rendering it more useful in certain circumstances.For example, by operating on a feed salinity of about 3.33% at an about67% recovery, a concentrate stream can be obtained with a concentrationof about 10% salt. In the case where predominantly monovalent ions areselectively transferred to said concentrate stream, the resulting streamcan be a predominantly pure monovalent (e.g., sodium chloride) stream ata concentration of about 10%. Such as stream can be used in part toregenerate ion exchange columns that have become exhausted, for use asbrine cleaning agents, for feed to crystallizers to efficiently producecrystalline salts, and/or in further electrochemical processes toproduce, for example, chlorine and caustic for disinfection or pHadjustment. Further, the concentrated salt can be cycled to theelectrolyte compartments of the ED device itself, and by-productchlorine and caustic, can be produced without the need for a separatecaustic/chlorine generation system, and avoid the need to provide addedchemicals other than salts already in the feed water to be desalted.

The monovalent species removal stage can utilize membranes thatselectively remove monovalent cations, monovalent anions, or bothmonovalent anions and cations. If it is desirable to produce pure sodiumchloride from a feed comprising calcium and sulfate salts, then thesystem can comprise both monovalent selective anion membranes and cationmembranes. Alternatively, if the objective is only to produce aconcentrate comprising pure sodium ions without concern for sulfatelevels, then the system can comprise only monovalent selective cationmembranes

The partially desalted product from the monovalent selective removalstage can then be sent to a divalent-reducing stage which can compriseany unit operation that reduces the concentration of divalent speciessuch as, but not limited to, calcium and magnesium salts. Examples ofunit operations that can serve to reduce the concentration of suchhardness causing species include, but are not limited to, ion exchangedevices, in particular, cation exchange columns utilizing cationexchange media. In addition, ion exchange media incorporating selectiveabsorbents and anion selective media can be used to selectively removeproblematic trace ions from the water, such as residual boron andbicarbonate, as well as divalent anions, such as sulfate. For selectiveabsorbents that require not only brine regeneration steps but also acidor caustic regeneration steps, the acid and caustic can optionally alsobe manufactured from the concentrated pure salt solution from a firststage concentrate effluent from a first monoselective membrane stage.

Embodiments represented by FIG. 2 further illustrate utilization of abyproduct stream from one stage in another stage to facilitate operationthereof. As exemplarily shown, the monovalent reducing stage can reducethe sodium chloride concentration of the water to be treated and collectsuch species in a concentrated byproduct stream which would typically bedischarged as a sodium chloride rich waste stream. This byproduct streamcan be utilized to regenerate the cation exchange media in thedivalent-reducing stage. The final stage can be considered as apolishing stage that further reduces the concentration of anyundesirable species and renders the water as potable. The byproductstream from this stage can be reintroduced or mixed with water to betreated or discharged. Thus, the illustrated embodiment canadvantageously reduce the overall discharge burden. In some instances,it would be impractical to utilize such a concentrate stream toregenerate the divalent removal stage, but because the monovalentselective devices typically operate at an elevated concentration, thepossibility of efficient regeneration of ion exchangers becomespossible. Also possible is the production of acid, caustic, andchlorine, for cleaning, sanitization, disinfection, and for aid toregeneration of specially selective ion exchangers such as boronselective ion exchange media.

The product water from the monoselective membrane device may be used forcertain purposes directly without need for further treatment, such asfor water for agriculture use that beneficially maintains a certainlevel of divalent ions relative to monovalent ions. Alternately, productwater from the second stage may be used directly, for example, where theproduct water is about 90% desalted and where the water is free or has areduced level of trace elements and divalent species. Alternatelyhowever, the product from the second stage can be sent to a thirdmembrane separation device comprising, for example, non-selective ED orEDI membranes, where in the water is further desalted to high levels ofpurity. In such a case, the concentrate solution from the third stagetypically contains essentially only one type of monovalent ions and thusthere is low potential for the third stage to scale or foul, and, theconcentrate, at high recovery can be recycled, at, for example, aconcentration similar to the source water, to feed the concentratestream of the first stage of the monovalent selective device. The endresult is an overall process that can provide various types of watersfor different end uses, while the system is operated under highlyefficient conditions that are not prone to fouling or scaling, where thewater recovery is much higher than traditional desalination techniques,and where the needed ancillary chemicals for regeneration, for removalof trace elements, for disinfection, for pH adjustment, and for cleaningare provided from the ionic makeup of the source water.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the spirit and scope ofthe invention. Accordingly, the foregoing description and drawings areby way of example only.

1. A method of reducing total dissolved solids in seawater comprising:reducing a concentration of dissolved solids in the seawater in ananofiltration device to produce a first partially treated water;reducing a concentration of dissolved solids in the first partiallytreated water in an electrodialysis device to produce a second partiallytreated water; and reducing a concentration of dissolved solids in thesecond partially treated water in an electrodeionization device toproduce treated water having less than about 1,000 ppm total dissolvedsolids and a boron content of less than about 0.5 ppm, wherein energyconsumption to produce the treated water through the nanofiltrationdevice, the electrodialysis device, and the electrodeionization deviceis less than about 7 kwh/kgal.
 2. The method of claim 1 wherein thenanofiltration device is operated at an upstream pressure of less thanabout 600 psi.
 3. The method of claim 1 wherein the treated watercomprises less than about 500 ppm total dissolved solids.
 4. The methodof claim 1, further comprising mixing a concentrate stream from theelectrodeionization device with the seawater upstream of thenanofiltration device.
 5. The method of claim 1, wherein reducing theconcentration of dissolved solids from the first partially treated watercomprises producing the second partially treated water from anelectrodialysis device having monovalent selective membranes.
 6. Themethod of claim 1, wherein reducing the concentration of dissolvedsolids in the seawater in a nanofiltration device comprises reducing aconcentration of monovalent species in the seawater stream to producethe first partially treated water product.
 7. The method of claim 1,wherein the treated water has more than about 230 ppm total dissolvedsolids.